PHOTOGRAPHIC PROCESSES. 75 



CHAPTER Y. 



PHOTOGRAPHIC PROCESSES. 



FROM among the many processes used in photogra- 

 phy we select the two best known, and most gener- 

 ally practised, referring the reader for further details 

 to works devoted to that subject. 



The wet collodion process will first claim our 

 attention, not only on account of the priority of its 

 invention, but because the gradations of the resulting 

 image and the fineness of the deposit give a beauty 

 to the results, hitherto unapproached by any other 

 method. Unfortunately on account of the lengthen- 

 ed exposure required, its employment in Photo- 

 micrography with high powers, is only possible when 

 sunlight is the source of illumination ; but with low 

 powers, and when an exposure of not more than ten 

 minutes is necessary, it is generally applicable. 



The size of plate required for Photo-micrography 

 is the carte-de-visite or quarter plate; the best ob- 

 tainable are made of imprimatur glass. 



If used before for the production of a negative, 

 the old picture can be removed from the glass by 

 warm water and a soft rag, then washed in a strong 

 solution of caustic potash to remove grease ; dipped 

 in water again several times, next in nitric acid, and 

 finally left soaking in a dish of water acidulated with 

 nitric acid. 



